1. Field of the Invention
The present invention relates to an image display adjusting device for allowing a display device such as a liquid crystal display to improve response characteristics in image display and display a moving image of high image quality.
2. Description of the Related Art
Flat panel displays (referred to as FPDs hereafter) of recent years are getting larger and higher-resolution, and a liquid crystal display is also demanded to be larger and of higher image quality. Of the FPDs, the liquid crystal display is particularly familiarized and drawing the highest attention. Therefore, there is a further demand for higher image quality. However, the liquid crystal display has a problem that its response speed of display is slower than that of other FPDs.
The response characteristics of a liquid crystal panel of the liquid crystal display will be presented hereunder. FIG. 28A shows a voltage waveform inside a liquid crystal layer, and FIG. 28B shows the voltage waveform after improving the response speed.
The liquid crystal panel changes an orientation of liquid crystal molecules by applying a voltage between liquid crystal layers according to a gradation to be displayed, and thereby controls a transmitted light volume of backlight so as to display an image. Here, an applied voltage for assigning intensity levels requires plenty of time before reaching a target gradation voltage due to factors such as a liquid crystal capacity, a CR time constant of connection resistance with a liquid crystal driving circuit and the like as shown in FIG. 28A. This is a cause of slowness of the response characteristics as to the liquid crystal panel. The slowness of the response characteristics is particularly conspicuous as to a moving image sequence in comparison with a conventional CRT and the like, where it remains as an afterimage. There is also a problem that the response characteristics are not equal among the liquid crystal panels.
A level adaptive overdrive (referred to as LAO hereafter) driving method is known as one of generally used techniques for improving the response characteristics. The LAO driving method supplies the liquid crystal panel with a higher driving voltage or a lower driving voltage than the gradation voltage of current frame data and thereby reduces a rise time or a fall time of the data so as to improve responsiveness. Here, an example of a general formula of improvement data by the LAO driving method is shown below.LAO=α(f1−f1)+f1  Formula (1)
Here, LAO: improvement data, α: highlight coefficient, f0: previous frame data, and f1: current frame data.
The formula (1) multiplies a difference value between a current frame and a previous frame by the highlight coefficient α, and adds the data after the multiplication to the current frame data as correction data for improving the response speed. It is thereby possible to acquire the improvement data having the response speed of the liquid crystal improved in a pseudo manner. As shown in FIG. 28B, this temporarily adds the correction data of a higher level or a lower level than a target gradation level on a rise or a fall of a liquid crystal driving waveform so that the time before reaching the target gradation level can be reduced. Such a LAO driving method is introduced as a heretofore known example by Japanese Patent Laid-Open No. 7-20828 and the like.
As for the technique of the LAO driving method, however, there arises a problem that image quality is degraded in the case where a specific image (such as a moving image) is displayed. This will be described below. In the description, [dec] denotes a decimal number and [hex] denotes a hexadecimal number.
As an example thereof, a case of image degradation due to over-highlight will be described with reference to FIGS. 29A and 29B. In the case where, as shown in FIG. 29A, there is a display Q of a gradation level 255 [dec] in a display P of a background gradation level 127 [dec] and the display Q of the gradation level 255 [dec] moves as in FIG. 29B, the gradation level of the data at a position before the movement in a screen after the movement (1 frame later) should be 127 [dec] as gradation data thereof. As the improvement in the responsiveness of the aforementioned formula (1) is implemented by the LAO, however, the data LAO on the improvement in the responsiveness of the image at the position before the movement in the screen after the movement becomes the LAO=α(127−255)+127. Here, the LAO depends on the value of the highlight coefficient α. However, α=0.5 is an optimal value in the other images, and so this value is also used as a fixed value here. In that case, it becomes LAO=0.5×(127−255)+127=63 [dec]. Therefore, the gradation level of the display Q at the position before the movement in the screen after the movement of FIG. 28B is 63 [dec] according to the foregoing calculation, which causes a problem that the gradation level lowers as against the background gradation level 127 [dec] and it significantly darkens. To be more specific, the image quality is degraded because the highlight coefficient α is overly set (overly highlighted) . Thus, setting of the value α also depends on a type of a display image and a characteristic of each individual display panel. If the value α is fixed, there is an adverse effect on the moving image described above as to the gradation level.
Japanese Patent Laid-Open No. 2005-173525 also describes an example using the LAO driving method. It describes that a highlight conversion parameter (OS parameter) equivalent to the value α is stored in an ROM for each individual gradation of image data so as to read out and use the parameter stored in the ROM according to the level of the image data.
In the example described in Japanese Patent Laid-Open No. 2005-173525, however, a circuit scale is expanded by using the ROM while it requires work of measuring the response characteristics of each individual liquid crystal display panel and deciding the parameter to the ROM, which takes a lot of trouble. It also has a drawback that the ROM size becomes larger and the circuit scale increases if rendered versatile to be adaptable to any panel.